<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .box {
            width: 100px;
            height: 100px;
            background-color: red;
        }

    </style>
</head>
<body>
    <div class="box"></div>

    <script>

        var box = document.querySelector('.box');

        // set: 设置
        // attribute: 属性
        // 元素.setAttribute('属性名', '属性值');
        box.setAttribute('style', 'background-color: blue; outline: 1px solid red;');

    </script>
</body>
</html>